PTGS2 and gastric cancer: Because most of the 31 pairs of GC samples were collected at 1–4 years ahead of diagnosis (n = 22) and GC diagnosis year (n = 21), we compared two groups and found that COX-2 methylation levels were significantly lower in GC diagnosis year samples than in 1–4 years pre-GC samples (1.5 % vs.2.5 %, p = 0.02).